January 2021
Intermediate
240 pages
5h 32m
Japanese
Content preview from 図解まるわかり データベースのしくみ
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84
グループ化したデータに
絞り込み条件を指定する
3-19
3-18で紹介した「GROUPBY」でグループ化した結果に対して、さら
に絞り込み条件を指定することができます。例えば本の情報が保存された
テーブルから、レコードを登録日ごとにグループ化して、日付ごとに入荷
した本の点数を集計することができますが、さらに条件を加えて指定した
日付に該当する結果のみを抽出するといったことが可能です。このように
ことが
できます。
絞り込み条件の追加には「HAVING」を使います。図3-42 は「users」
テーブルの「gender」カラムでグループ化してグループごとのレコード数
を集計し、そこからレコード数が3 以上の結果のみを絞り込んでいる例で
す。「HAVING」の後ろにグループ化で集計した結果の絞り込み条件を追加
したので、最終的に「man」のレコードが 3 件という結果のみを取得して
います。
「WHERE」と「HAVING」は、検索条件を指定するという意味では使い
方が似ていますが、実行される順番に違いがあります。「」
「」
されます。ユーザー情報が保存されている
テーブルから、男性ユーザーが3 人以上登録されている年齢を集計したい
場合、図3-43 のようなコマンドになります。処理の順番は以下の通りで
す。
❶「
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.